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[Tabs] Give MDCTabBarDelegate pass through methods for "willDisplayCe…
…ll"/"didEndDisplayingCell" (#7518) One of the solutions proposed in #6275 is to provide pass through methods for the UICollectionViewDelegate methods `-collectionView:willDisplayCell:forItemAtIndexPath:` and `-collectionView:didEndDisplayingCell:forItemAtIndexPath:`. This is the one I chose, mainly because it seemed the most straightforward. [One comment](https://github.com/material-components/material-components-ios/issues/6275#issuecomment-453650714) said we should be careful about this approach, and [another](https://github.com/material-components/material-components-ios/issues/6275#issuecomment-454204635) said we needed to look more into the "pre-fetching" behavior of these methods. It seems like a good approach to me... Scrolling through the tabs in the TabBarTextOnlyExample with print statements seems to work how I would expect it to. 👍
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,46 @@ | ||
| // Copyright 2019-present the Material Components for iOS authors. All Rights Reserved. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
|
|
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| #import <Foundation/Foundation.h> | ||
| #import "MDCTabBar.h" | ||
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| /** | ||
| An additional delegate protocol for MDCTabBar that provides information about when UITabBarItems | ||
| are about to be displayed and when they stop being displayed. | ||
| */ | ||
| @protocol MDCTabBarDisplayDelegate | ||
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| /** | ||
| This method is called sometime before the tab's view is displayed. | ||
| */ | ||
| - (void)tabBar:(nonnull MDCTabBar *)tabBar willDisplayItem:(nonnull UITabBarItem *)item; | ||
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| /** | ||
| This method is called sometime after the tab's view has stopped being displayed. | ||
| */ | ||
| - (void)tabBar:(nonnull MDCTabBar *)tabBar didEndDisplayingItem:(nonnull UITabBarItem *)item; | ||
|
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| @end | ||
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| @interface MDCTabBar (MDCTabBarDisplayDelegate) | ||
|
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| /** | ||
| A delegate that allows implementers to receive updates on when UITabBarItems are about to be | ||
| displayed and when they stop being displayed. | ||
| @note This property may be removed in a future version and should be used with that understanding. | ||
| */ | ||
| @property(nonatomic, weak, nullable) NSObject<MDCTabBarDisplayDelegate> *displayDelegate; | ||
|
|
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| @end |
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components/Tabs/tests/unit/MDCTabBarDisplayDelegateTests.m
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| // Copyright 2019-present the Material Components for iOS authors. All Rights Reserved. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
|
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| #import <XCTest/XCTest.h> | ||
|
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| #import "MDCItemBar.h" | ||
| #import "MDCTabBarDisplayDelegate.h" | ||
| #import "MaterialTabs.h" | ||
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| @interface MDCTabBarDisplayDelegate : NSObject <MDCTabBarDisplayDelegate> | ||
| @property(nonatomic, assign) BOOL willDisplayItemWasCalled; | ||
| @property(nonatomic, assign) BOOL didEndDisplayingItemWasCalled; | ||
| @end | ||
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| @implementation MDCTabBarDisplayDelegate | ||
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| - (void)tabBar:(MDCTabBar *)tabBar willDisplayItem:(UITabBarItem *)item { | ||
| self.willDisplayItemWasCalled = YES; | ||
| } | ||
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| - (void)tabBar:(MDCTabBar *)tabBar didEndDisplayingItem:(UITabBarItem *)item { | ||
| self.didEndDisplayingItemWasCalled = YES; | ||
| } | ||
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| @end | ||
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| @interface MDCTabBarDisplayDelegateTests : XCTestCase | ||
| @end | ||
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| @implementation MDCTabBarDisplayDelegateTests | ||
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| - (void)testMDCTabBarDisplayDelegateTabBarWillDisplayItemWhenViewHasBeenLaidOut { | ||
| // Given | ||
| MDCTabBar *tabBar = [[MDCTabBar alloc] initWithFrame:CGRectZero]; | ||
| MDCTabBarDisplayDelegate *displayDelegate = [[MDCTabBarDisplayDelegate alloc] init]; | ||
| tabBar.displayDelegate = displayDelegate; | ||
| CGFloat tabBarHeight = [MDCTabBar defaultHeightForItemAppearance:tabBar.itemAppearance]; | ||
| tabBar.frame = CGRectMake(0, 0, 200, tabBarHeight); | ||
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| // When | ||
| UITabBarItem *item1 = [[UITabBarItem alloc] initWithTitle:@"first tab" image:nil tag:0]; | ||
| UITabBarItem *item2 = [[UITabBarItem alloc] initWithTitle:@"second tab" image:nil tag:0]; | ||
| tabBar.items = @[ item1, item2 ]; | ||
| [tabBar setNeedsLayout]; | ||
| [tabBar layoutIfNeeded]; | ||
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| // Then | ||
| XCTAssertTrue(displayDelegate.willDisplayItemWasCalled); | ||
| } | ||
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| - (void)testMDCTabBarDisplayDelegateTabBarDidEndDisplayingItemWhenViewHasBeenLaidOut { | ||
| // Given | ||
| MDCTabBar *tabBar = [[MDCTabBar alloc] initWithFrame:CGRectZero]; | ||
| MDCTabBarDisplayDelegate *displayDelegate = [[MDCTabBarDisplayDelegate alloc] init]; | ||
| tabBar.displayDelegate = displayDelegate; | ||
| CGFloat tabBarHeight = [MDCTabBar defaultHeightForItemAppearance:tabBar.itemAppearance]; | ||
| tabBar.frame = CGRectMake(0, 0, 200, tabBarHeight); | ||
| UITabBarItem *item1 = [[UITabBarItem alloc] initWithTitle:@"first tab" image:nil tag:0]; | ||
| UITabBarItem *item2 = [[UITabBarItem alloc] initWithTitle:@"second tab" image:nil tag:0]; | ||
| tabBar.items = @[ item1, item2 ]; | ||
| [tabBar setNeedsLayout]; | ||
| [tabBar layoutIfNeeded]; | ||
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| // When | ||
| UITabBarItem *item3 = [[UITabBarItem alloc] initWithTitle:@"third tab" image:nil tag:0]; | ||
| UITabBarItem *item4 = [[UITabBarItem alloc] initWithTitle:@"fourth tab" image:nil tag:0]; | ||
| tabBar.items = @[ item3, item4 ]; | ||
| [tabBar setNeedsLayout]; | ||
| [tabBar layoutIfNeeded]; | ||
|
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| // Then | ||
| XCTAssertTrue(displayDelegate.didEndDisplayingItemWasCalled); | ||
| } | ||
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| @end |